Conventionally, in construction of a precast piling by a preboring method, generally, by lowering a screw rod attached to an auger machine that moves up and down along a leader of a three-point pile driver, etc., while driving and rotating the screw rod, the ground is dug and a piling hole is formed to have a depth reaching an estimated bearing stratum of deep underground, and thereafter, while the screw rod is pulled up, a foot protection liquid such as soil cement is injected into the piling hole, and subsequently, a hole circumference fixative solution is injected up to an upper portion of the piling hole, and this screw rod is pulled out, and then a precast piling such as a PHC piling or PRC piling is connected to the auger machine A via a connecting rod, and this precast piling is inserted and embedded in the piling hole.
On the other hand, for construction of a cast-in-place piling, a kelly bar type earth drilling method is widely used. In this construction method, a cast-in-place piling is erected through the process of digging the ground and removing soil by a shaft digging bucket connected to a kelly bar of an earth drill machine, press-fitting a standpipe in the dug hole, and while injecting a slurry such as a bentonite solution into the hole, digging the ground and removing soil by a replaced shaft digging bucket with a small diameter to form a piling hole with a depth reaching an estimated bearing stratum of deep underground, and inserting, in this piling hole, a reinforced frame and a tremie pipe, and after discharging slime by air introduction, placing concrete and pulling out the tremie pipe and the standpipe. It is noted that, as belled pile construction, a cast-in-place piling is often erected after a bottom portion of a piling hole is enlarged by a belling bucket. As other cast-in-place piling construction methods, there are also known methods such as an all casing method in which after a casing is pressed into the ground while being repeatedly rotated, soil inside is dug with a hammer grab, and after a reinforced frame is inserted into a hole, concrete is placed while the casing is pulled out, and a reverse construction method in which the ground is dug by rotating a drill bit, and a hole is bored by discharging dug soil onto the ground together with water inside the hole, a hole wall is protected by circulating, inside the hole, water separated from soil, and a reinforced frame is erected inside the hole and concrete is placed.
Generally, whether or not a piling support force after completion of erection by these construction methods is sufficient can be determined by a load carrying test, however, when insufficient support strength is found by this test, large amounts of labor, time, and cost are needed for redoing piling construction, it is therefore ideal if it can be determined whether or not a support force of a dug piling hole is sufficient for a pile distal end before erecting the piling. Conventionally, from such a viewpoint, a method has been proposed in which a change in digging load is detected from a current value of a rotary drive motor of a digging member, reaching a piling hole to a hard bearing stratum of deep underground is confirmed based on an increase in digging load (Japanese Published Unexamined Patent Application No. H5-280031, Japanese Published Unexamined Patent Application No. 2000-245058 and Japanese Published Unexamined Patent Application No. 2003-74045).
However, in the method in which a change in digging load is detected from a current value of a rotary drive motor, as a dug hole becomes deeper, as frictional resistance between the digging member and the hole wall increases, the digging load significantly increases even when the hole does not reach a bearing stratum, and to the contrary, due to slippage occurring at a digging portion, the digging load decreases although the hole has reached a bearing stratum, and further, the digging load greatly fluctuates depending on an operator's digging operation skill, therefore, the change in digging load is poor in reliability as an index of confirmation of reach to a bearing stratum. Therefore, generally, exclusively based on data of a geological sample obtained through pilot boring at a planned construction site, digging to a prescribed depth is usually regarded as reaching a bearing stratum, however, the deep underground of the entire planned construction site is not always uniform in stratigraphic succession and homogeneous, and depending on geological history, there are many cases where the depth of the bearing stratum locally differs or the hardness of the bearing stratum greatly differs, therefore, it cannot be said that bottom portions of individual piling holes do not have sufficient support forces for piling distal ends in actuality.